1. Field of the Invention
The present invention relates to a structure of a claw-pole stepping motor with rotor including a vibration reducing magnet to be used as the spindle motor for information storage apparatuses represented by FDD, HDD, CD-ROM, MO, MD, and the like.
2. Description of the Related Art
As the spindle motors for storage apparatuses, three phase brushless DC motors have so far been used from the point of view of the performances required of them. However, it has become difficult for the motors to meet the recent demands for lower prices, and therefore studies have begun on the use of claw-pole stepping motors as lower cost substitutes. Since the motor of this type is a synchronous motor, the motor control can be executed in the open-loop system instead of the previously practiced closed-loop system and, therefore, the control circuitry can be greatly simplified.
FIG. 7 is a sectional view of a conventional outer rotor type stepping motor 7, the structure of which will be described below. The stepping motor 7 is installed on a base plate 10. The stepping motor has two stator yokes 11 and 12 disposed on the base plate 10 one above the other. Within the stator yokes 11 and 12, there are disposed stator coils 13 and 14, respectively, and the interior of the stator yokes 11 and 12 is shaped into a cylindrical form. In the cylindrical space, there is disposed a bearing 17 upright from the base plate 10 and a revolving shaft 18 is rotatably borne by the bearing 17. The bottom end of the bearing 17 is rotatably supported by a thrust bearing 19 on the base plate 10.
On the outer side walls of the stator yokes 11 and 12, there are formed pole teeth 15 and 16. On the top end of the revolving shaft 18, there is mounted a cup-shaped rotor 20, and the rotor 20 has rotor magnets 21 fixed at portions thereof opposing the pole teeth 15 and 16. At the upper portion of the rotor 20, i.e., at the bottom portion of the cup shape, a chucking mechanism 8 is formed to retain the chucking hub of a disk. Reference numeral 22 denotes a pin to be fitted into a hole in the hub of a disk 5 (see FIG. 1a)
As to the positional relationship of the stator coils 13 and 14 and the stator yokes 11 and 12 with the rotor magnets 21 of the claw-pole stepping motor, the stator A and the stator B are disposed one above the other as shown in FIG. 7 and they are excited alternately. By the alternate excitation of the stator A and stator B, there arises a vertical motion as well as a revolving motion of the rotor 20. When the stator A is excited, the rotor magnets 21 receive a force pulling them as a whole upward in the axial direction as well as a rotating force and, when the stator B is excited in the next excitation pulse, the rotor magnets 21 receive a force pulling them downward in the axial direction as well as a rotating force. Namely, by the application of the pulses, the rotor 20 is caused to move up and down in the axial direction as well as to rotate.
The vertical movement becomes a vibration at higher pulse rates as the revolution of the motor increases. When the motor is used as the spindle motor of an information recording disk, the vibration causes surface deviation and also produces uneven rotation. Further, since there are stator coils 13 and 14 within the stator A and stator B and these coils are formed in a solenoidal form around the shaft, a great flux leakage in the axial direction is produced around the revolving shaft by the coil excitation. The flux leakage is superposed as a noise on the output of the recording and reproducing head and causes a deterioration in the S/N ratio of the signal.
The present invention has been made under the above described situation and provides a rotor structure whereby the vibration of the revolving shaft in the axial direction of the claw-pole stepping motor as well as the flux leakage are suppressed.